Abstract Functional genome wide association studies have indicated that dysregulated microglia activation is a major contributing factor for neurodegeneration and cognitive decline in Alzheimer's disease (AD). Thus, regulating the microglia activation state could have profound therapeutic potential for AD. However, the underlying mechanisms by which microglia transition between different states during normal and disease conditions are largely unknown. S1P-signaling has been implicated to play important roles in AD. Yet the function of S1P in AD is not clear due to conflicting reports. On one hand, the level of S1P is reduced in postmortem AD patient brains, suggesting a protective role of S1P. On the other hand, there are also reports indicating that S1P might play the opposite. Although has been tested in A?-related AD mouse models and showed promising results, the role of FTY720 in tau pathology is elusive. Further, FTY720 can function as an agonist for all S1PRs except S1PR2, as a functional antagonist for S1PR1, or as a ceramide mimic depending on the cellular context and treatment procedure, which makes interpretation of S1P's role challenging. Thus, the function of S1P-signaling in AD is more complex than assumed and demands further investigation Our preliminary studies suggest that Spinster homolog 2 (Spns2), an S1P transporter, critically modulates microglial transition from inflammatory to anti- inflammatory states when treated with A? peptide. This data is exciting in that it reveals microglia states could be regulated through Spns2 and/or S1P. Our goal in this application is to define the function and underlying mechanism of Spns2 in microglial responses in AD. Our overarching hypothesis is that Spns2 contributes to AD pathogenesis by promoting microglial pro- inflammatory activation induced by A? and tau. By focusing on the S1P transporter Spns2, this proposal holds a unique premise to reveal novel aspects of S1P-signaling in AD. We will (1) test that Spns2-mediated S1P transport enhances inflammatory responses in microglia in AD models in vitro, (2) determine the downstream signaling mediated by Spns2/S1P in inflammatory response in AD models in vitro, and (3) test that Spns2KO ameliorates AD-related phenotypes in a murine AD model. By focusing on the S1P transporter Spns2 in microglia biology, this proposal holds a unique premise to reveal novel aspects of the function of Spns2 and S1P in AD.